This paper presents a novel method for realistically characterizing discontinuity systems for underground excavations. The basic idea is to sample and simulate discontinuities at different scales. A combined sampling technique, which consists of axionometric projection and 3D-digital images, makes possible the efficient acquisition of detailed discontinuity data. Minor discontinuities are simulated stochastically based on their representative orientation statistics, with size parameters estimated using analytical methods for circular window sampling. Censored major discontinuities are simulated conditionally using actual intersection locations and true orientations, with discontinuity sizes first assessed using stochastic forward modeling. This method has been applied to the Raabstollen adit, part of an underground mine complex in eastern Styria, Austria.
Discontinuity pattern assessment is often a key issue for underground excavations (Hudson, 1991), but the complete three-dimensional description of discontinuities is inherently data limited. Nevertheless, for engineering applications, it is necessary to describe the overall discontinuity system, which can be treated as simplified mathematical representations of discontinuity geometries (Dershowitz 1984).
Increasing the quality of discontinuity system representation with more advanced and efficient means is currently being addressed in the context of rock mechanics and rock engineering (e.g. Jing, 2003, Pine et al. 2006). With the recent improvements in remote three-dimensional photogrammetric systems (e.g. Gaich 2001, Fasching 2001) and advanced discontinuity modeling technologies such as FracMan (Dershowitz 1995, Rogers et al. 2006, 2007), it is becoming possible to assess discontinuity patterns with more confidence. It is the author's experience that besides discontinuity type, orientation, etc., the actual discontinuity location and size are crucial.
In this paper we summarize an approach for realistically characterizing discontinuity systems for underground excavation. As a case study, the method is applied to the Raabstollen adit, part of the Arzberg underground mining complex in eastern Styria, Austria.
Host rocks in the Raabstollen adit are mainly chloritic schists. Within a 31.5m length of the adit is a relatively uniform schist unit and this location was chosen for discontinuity system modeling. The horse-shoe shaped adit has the cross-sectional area of about 2m×2m. Discontinuity sampling was performed along the tunnel ribs and crown area.
Observations of discontinuity traces showed that there are five main discontinuity sets. The schistosity set is predominantly gently dipping to flat-lying. The slickensided set has very little attitude dispersion and the largest trace length, which usually takes several meters. The others are joint sets (J1, J2, and J3) with relatively shorter trace length.
Larger slickensides and small faults within the selected mapping interval usually intersect more than one excavation surface, and in the small adit it proved difficult to include the entire trace of a larger discontinuity in a single 3D digital image photo. It has been shown (e.g. Schubert 2001) that large discontinuities control the displacement pattern of the tunnel periphery. Therefore, large discontinuities should be characterized deterministically, with an unbiased representation in the eventual discontinuity model. We applied an unrolled tunnel mapping technique based on axionometric projection, as depicted schematically in Figure 1.
